Minor planet moon
A minor planet moon is a minor planet that orbits another minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size. Discoveries of asteroid moons (and binary objects, in general) are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights of their physical properties that is generally not otherwise possible. Terminology In addition to the terms satellite and moon, the term binary is sometimes used for minor planets with moons (or triple for minor planets with two moons). If one object is much bigger it is usually referred to as the primary and its companion as secondary. The term double asteroid is sometimes used for systems in which the asteroid and its moon are roughly the same size, while binary tends to be used independently from the relative sizes of the components. Discovery milestones As early as 1978, following a stellar occultation, 532 Herculina had been suggested to have a moon and there were reports of other asteroids having companions (usually referred to as satellites) in the following years. A letter in Sky & Telescope magazine at this time pointed to pairs of large craters (e.g. the Clearwater Lakes in Quebec) also suggesting asteroids having companions. However, it was not until 1993 that the first asteroid moon was confirmed when the Galileo probe discovered Dactyl orbiting 243 Ida. The second was discovered around 45 Eugenia in 1998. The first Trans-Neptunian binary, was optically resolved in 2002.Chiang, E.; Lithwick, Y.; Buie, M.; Grundy, W.; Holman, M.; A Brief History of Trans-Neptunian Space, to appear in Protostars and Planets V (August 2006) Final preprint on arXiv , 104 asteroid moons had been discovered, 60 in the main belt, 2 orbiting Trojan asteroids, 42 near-Earth objects and Mars-crossers. There were at that time also 58 Trans-Neptunian moons. In 2005, the asteroid 87 Sylvia was discovered to have two moons, making it the first known triple asteroid. This was followed by the discovery of a second moon orbiting 45 Eugenia. Also in 2005, the KBO was discovered to have two moons, making it the second KBO after Pluto known to have more than one moon.An example of a double asteroid is [[90 Antiope], where two roughly equal-sized components orbit the common centre of gravity. 617 Patroclus and its same-sized companion Menoetius is the only known binary system in the Trojan population. Commonality The data about the populations of binary objects are still patchy. In addition to the inevitable observational bias (dependence on the distance from Earth, size, albedo and separation of the components) the frequency appears to be different among different categories of objects. Among asteroids, an estimated 2% would have satellites. Among trans-Neptunian objects (TNO), an estimated 11% are believed to be binary or multiple objects, but three of the four known large TNO (75%) have at least one satellite. More than 20 binaries are known in each of the main groupings: Near Earth asteroids, Main belt asteroids, and Trans-Neptunians, not including numerous claims based solely on the light curve variation. No binaries have been found so far among Centaurs with semi-major axis smaller than Neptune.Noll, Keith S. "Solar System binaries", Asteroids, Comets, Meteors, Proceedings of the 229th Symposium of the IAU, Rio de Janeiro, 2005, Cambridge University Press, 2006., pp.301-318 Preprint However, using an extended definition of Centaurs, as the objects on instable orbits with the perihelion inside the orbit of Neptune, the first binary Centaur 42355_Typhon (previously known as ) was identified in 2006. Origin The origin of asteroid moons is not currently known with certainty, and a variety of theories exist. A widely accepted theory is that asteroid moons are formed from debris knocked off of the primary asteroid by an impact. Other pairings may be formed when a small object is captured by the gravity of a larger one. Formation by collision is constrained by the angular momentum of components i.e. by the masses and their separation. Close binaries fit this model (e.g. Pluto/Charon). Distant binaries however, with components of comparable size, are unlikely to have followed this scenario, unless considerable mass has been lost in the event. The distances of the components for the known binaries vary from a few hundreds of kilometres (243 Ida, 3749 Balam) to more than 3000 km (379 Huenna) for the asteroids. Among TNOs, the known separations vary from 3,000 to 50,000 km. Populations What is "typical" for a binary asteroid system tends to depend on its location in the Solar System (presumably because of different modes of origin and lifetimes of such systems in different populations of minor planets). * Among Near-Earth Asteroids, satellites tend to orbit at distances of the order of 3-7 primary radii, and have diameters two to several times smaller than the primary. Since these binaries are all inner-planet crossers, it is thought that tidal stresses that occurred when the parent object passed close to a planet may be responsible for the formation of many of them, although collisions are thought to also be a factor in the creation of these satellites. * Among main belt asteroids, the satellites are usually much smaller than the primary (a notable exception being 90 Antiope), and orbit around 10 primary radii away. Many of the binary systems here are members of asteroid families, and a good proportion of satellites are expected to be fragments of a parent body whose disruption after an asteroid collision produced both the primary and satellite. * Among Trans-Neptunian Objects, it is common for the two orbiting components to be of comparable size, and for the semi-major axis of their orbits to be much larger − about 100 to 1000 primary radii. A significant proportion of these binaries are expected to be primordial. Dwarf planets Among the dwarf planets, it is 90 percent certain that has no moons larger than 1 km in size, assuming that they would have the same albedo as Ceres itself. Pluto has three moons. Its largest moon Charon, named after the ferryman who took souls across the River Styx, is more than half as large as Pluto itself, and large enough to orbit a point outside Pluto's surface. In effect, each orbits the other, forming a binary system informally referred to as a double-dwarf-planet. Pluto's two other moons, Nix and Hydra, are far smaller and orbit the Pluto–Charon system. has no known moons. A satellite having 1% Makemake's brightness would have been detected if it had been located at an angular distance from Makemake farther than 0.4 arcseconds (0.0001 degrees). has one known moon, Dysnomia. Its radius, based on its brightness, is estimated to be 50 to 125 km. Haumea has two small moons with radiuses estimated around 155 km (Hiʻiaka) and 85 km (Namaka). List of minor planet moons Near Earth objects Mars crossers Main belt asteroids Jupiter trojans Trans-Neptunian objects See also *Yarkovsky-O'Keefe-Radzievskii-Paddack effect (YORP effect) References *''Asteroids with Satellites'' web page, maintained up to date by W. Robert Johnston; and references therein. (last accessed 13-03-2007) *''The VOBAD database '' a web page built and designed by F. Marchis and his collaborators (UC-Berkeley/SETI Institute) which contains the parameters of 169 multiple asteroid systems (last update May 9 2009) External links *Orbits of Binary Asteroids with Adaptive Optics *Asteroids with Satellites Category:Asteroid satellites Category:Binary asteroids Category:Moons Category:Binary asteroids Category:Asteroid satellites Category:Lists of minor planets Category:Lists of moons bg:Астероиден спътник ca:Satèl·lit asteroidal cs:Asteroidní měsíc es:Satélite asteroidal fr:Lune astéroïdale it:Satellite asteroidale nl:Planetoïdemaan ja:小惑星の衛星 nn:Asteroidemåne pl:Księżyc planetoidy ru:Спутник астероида simple:Asteroid moon sk:Mesiac planétky sl:Asteroidna luna sv:Asteroidmåne th:ดวงจันทร์บริวารดาวเคราะห์น้อย zh:小行星衛星